Polystyrene foamed mold articles are being used as a heat-insulating material, a packaging fitment and cushioning material in various fields. However, the polystyrene foamed mold article has disadvantages that its recovery of compression strain is low, and that it is poor in heat resistance, which is 70.degree. to 80.degree. C. at the highest, and in oil resistance and impact resistance.
In an attempt to eliminate these disadvantages, there have been proposed foamed particles of propylene resins [see, JP-B-49-2183, JP-A-57-90027, JP-A-57-195131, JP-A-58-1732, JP-A-58-23834, JP-A-58-25334, JP-A-58-33435, JP-A-58-55231, JP-A-58-76229, JP-A-58-76231, JP-A-58-76232, JP-A-58-76233, JP-A-58-76234, JP-A-58-87027 and JP-A-62-151325 (The terms "JP-A" and "JP-B" as used herein mean an "unexamined published Japanese patent application" and an "examined Japanese patent publication", respectively)].
Of such foamed particles of propylene resins, the foamed particles composed of a propylene homopolymer as a base resin should be molded, with high-pressure steam, i.e., high-temperature steam, applied thereto so as to complete the melt-adhesion of the foamed particles. Thus, there has been proposed preliminarily foamed particles prepared by the use of an ethylene-propylene random copolymer, as a base resin, having an ethylene content of from 1 to 20% by weight (see, JP-B-59-23731).
The above-mentioned preliminarily foamed particles are advantageous in that they can be molded at a lower steam pressure than that for foamed particles composed of such a base resin as a propylene homopolymer or an ethylene-propylene block copolymer.
On the other hand, in the field of automobile bumpers, the conventional metallic bumpers are being replaced by plastic bumpers, particularly those composed of a core material of foamed resin and a surface material of non-foamed resin, in order to decrease the weight of an automobile. As raw resins for such a foamed bumper core material, there have been employed a polyurethane foam, polystyrene, crosslinked polyethylene and an ethylene-propylene random copolymer [see, "Foam Times" No. 842, published on July 25, 1981, JP-A-U-55-163254 (The term "JP-A-U" as used herein means an "unexamined published Japanese utility model application"), U.S. Pat. No. 4,350,378, JP-A-58-221745, JP-A-60 189660, and U.S. Pat. Nos. 4,504,534 and 4,600,636].
A bumper core material composed of a foamed mold article whose base resin is an ethylene-propylene-random copolymer having an ethylene content of from 1 to 12% by weight and which has a density of from 0.03 to 0.065 g/cm.sup.3 is superior in light weight properties to polyurethane foams, superior in dimensional recovery and heat resistance to polystyrene foamed mold articles, and superior in heat resistance and strength to crosslinked polyethylene foamed mold articles. For this reason, there has recently been a growing demand for this foamed mold article as a bumper core material.
Foamed particles of a propylene resin is produced by the process which comprises dispersing propylene resin particles into water in a closed vessel, feeding a volatile organic blowing agent into the closed vessel, heating the dispersion to a temperature above the softening point of the propylene resin particles while the pressure within the closed vessel is being kept at the vapor pressure of the blowing agent or a higher pressure, maintaining that temperature and pressure for a certain period of time, and then opening a discharge port provided in the closed vessel below the water level to release, together with the water, the propylene resin particles impregnated with the volatile blowing agent into an atmosphere having a lower pressure than that within the vessel (refer to the Japanese Patent Publications as mentioned hereinbefore).
In this process for preparing propylene resin foamed particles, an ethylene-propylene random copolymer can also be advantageously used as a raw resin. That is, in the case where foamed particles having a certain expansion rate are to be produced from an ethylene-propylene random copolymer having a lower melting point, a propylene homopolymer and an ethylene-propylene block copolymer, separately, with the filling ratio (the ratio of the total volume of the contents such as the water, resin, etc. to be used for the reaction to the capacity of the closed vessel) kept constant over the three raw resins, the heating temperature for the ethylene-propylene random copolymer having a lower melting point can be low and, hence, the inner pressure of the closed vessel can also be low as compared with those for the other two polymers, so that a closed vessel designed to have a lower pressure resistance can be employed for the ethylene-propylene random copolymer.
Because of its low melting point, an ethylene-propylene random copolymer having a higher ethylene content is preferred, from the viewpoint of easy foaming and molding, to an ethylene-propylene random copolymer with a lower ethylene content.
However, in order to improve the compressive strength of a foamed mold article (such as, for example, a bumper core material as disclosed in the aforementioned (JP-A-58-221745) obtained by heating and melt-adhering, in a mold, foamed particles comprising an ethylene-propylene random copolymer as a base resin, the expansion rate of the intended foamed mold article should be lowered as described in the aforementioned JP-A-60-189660, or an ethylene-propylene random copolymer having a low ethylene content should be employed as a base resin.
If such a foamed mold article is for use as a bumper core material, it is not preferable to lower the expansion rate, i.e., to increase the density, of the mold article (bumper core material) from the viewpoint of automobile design, because it results in an increased weight of the bumper core material.
Thus, in order to meet the need for lighter-weight automobile, a foamed mold article having a lower density is preferred over others if the former has the same strength as those of the latter.
On the other hand, if an ethylene-propylene random copolymer having a low ethylene content is employed as a base resin, the pressure resistance of the closed vessel to be used for preparing foamed particles from the base resin should be increased and the operating temperature also increased, as mentioned before, so as to cope with the high melting point of the base resin. Further, where the thus-prepared foamed particles are subjected to molding, the molding machine should be modified to increase its mold clamping force and there are other economical disadvantages related to equipment and service.
Moreover, the sizes of the cells of the foamed particles obtained from such ethylene-propylene random copolymer are so small that the mutual melt-adhesion of the foamed particles at the time of molding is not good.
JP-A-60-110734 disclosed preliminarily foamed particles of a propylene-1-butene random copolymer having a 1-butene content of from 15 to 40 mol % (19 to 47% by weight) and a heat of crystallization of from 5 to 15 cal/g.
The above-mentioned preliminarily foamed particles are molded into a foamed mold article by a process which comprises imparting an elevated pressure to the inside of the preliminarily foamed particles with pressurized air, filling the resulting particles into a cavity of a mold, and applying heat to the particles in the cavity of a mold to take place secondary foaming. The purpose of the use of the propylene-1-butene random copolymer in this process is to effect the heating at a lower temperature, and there is no description therein teaching or suggesting a method of improving the mechanical strength of the foamed mold article.
Since this propylene-1-butene random copolymer having a 1-butene content of from 15 to 40 mol % has a low crystallinity as apparent from its heat of crystallization as low as 5 to 15 cal/g, the molding can be effected at a lower temperature. However, the thus-obtained foamed mold article shows little improvement in compressive strength.